The present invention relates to a tubular jacket heater or, more particularly, to a tubular jacket heater for preheating tubular lamps employed as a light source in electrophotographic copying machines, facsimile communication apparatuses, microfilm reader-printers and the like.
In various image forming machines such as those named above, one of the important process steps is the exposure of respective photosensitive materials to light from a light source such as a fluorescent lamp, iodine lamp and the like. In order to ensure instantaneous starting of these lamps by turning the switches on and to obtain stabilization of the light intensity during operation, it is desirable that the lamps are kept at a constant temperature not so low even when the machine per se is not in operation and throughout the operation. For example, the optimum temperature for the stabilization of the light intensity in a fluorescent lamp is 40.degree. to 43.degree. C. and, therefore, a preheating means in the lamp is indispensable, especially, in winter.
Accordingly, it is usual that these lamps are provided with a tubular jacket heater with a C-wise radial cross section having an aperture extending in the direction parallel to the axis of the tubular lamp through which light is emitted and the temperature of the lamp is kept constant at the optimum operating temperature by the aid of a thermostat.
These tubular jacket heaters have a structure such that a thin, sheet-like heater element is sandwiched therein. An inner insulating co-act to provide a heater having a C-shape layer and an outer insulating layer. The material for these insulating layers must be selected with consideration of several factors. For example, the material should be sufficiently resistant to heat at a temperature of the surface of the lamp which sometimes reach 120.degree. C. or higher when the lamp is lighted and also fire-resistant even in an electrical accident along with a good workability to be shaped into a tubular form with a C-wise cross section.
In the prior art, the inner insulating layer of the tubular jacket heaters for preheating tubular lamps is made of a polycarbonate resin which satisfies the above requirements, on to which a heater element is bonded by use of an adhesive agent with an outer covering layer made of an insulating material such as glass cloth.
The above described tubular jacket heater with an insulating layer made of a polycarbonate resin is defective due to the lack of flexibility of the resin leading to an incomplete close contact of the inside surface of the heater and the surface of the lamp so that the heat efficiency of the tubular heater is relatively low. Consequently, the wattage of the heater element must be excessively large and an undesirably long time is taken for the elevation of the temperature from the ambient to the desired optimum preheating temperature of the fluctuation in the temperature of the lamp during operation is unduly large.